1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly to a photocurable organic material having an improved molecular density and a method of fabricating an organic insulating layer used for an array substrate for LCD device using the same.
2. Discussion of the Related Art
Since the LCD device has characteristics of light weight, thinness and low power consumption, the LCD device has been widely used as a substitute for a display device of cathode-ray tube type.
The LCD device uses optical anisotropy and polarization properties of liquid crystal molecules to display images. The liquid crystal molecules have orientation characteristics of arrangement resulting from their thin and long shape. Thus, an arrangement direction of the liquid crystal molecules can be controlled by applying an electrical field to them. Particularly, the LCD device including a thin film transistor (TFT) as a switching element, referred to as an active matrix LCD (AM-LCD) device, has excellent characteristics of high resolution and displaying moving images. Since the LCD device includes the TFT as the switching element, it may be referred to a TFT-LCD device.
Generally, the LCD device includes an array substrate, where a TFT and a pixel electrode are formed, a color filter substrate, where a color filter layer and a common electrode are formed, and a liquid crystal layer. The array substrate and the color filter layer face and are spaced apart from each other. The liquid crystal layer is interposed therebetween.
FIG. 1 is an exploded perspective view of a conventional LCD device. As shown in FIG. 1, the LCD device includes an array substrate 10, a color filter substrate 20 and a liquid crystal layer 30. The array substrate 10 and color filter substrate 20 face each other, and the liquid crystal layer 30 is interposed therebetween.
The array substrate includes a gate line 14, a data line 16, a TFT “Tr”, and a pixel electrode 18 on a first substrate 12. The gate line 14 and the data line 16 cross each other such that a region formed between the gate and data lines 14 and 16 is defined as a pixel region “P”. The TFT “Tr” is formed at a crossing portion between the gate and data lines 14 and 16, and the pixel electrode 18 is formed in the pixel region “P” and connected to the TFT “Tr”.
The color filter substrate 20 includes a black matrix 25, a color filter layer 26, and a common electrode 28 on a second substrate 22. The black matrix 25 has a lattice shape to cover a non-display region of the first substrate 12, such as the gate line 14, the data line 16, the TFT “Tr”, and so on. The color filter layer 26 includes first, second, and third sub-color filters 26a, 26b, and 26c. Each of the sub-color filters 26a, 26b, and 26c has one of red, green, and blue colors “R”, “G”, and “B” and corresponds to the each pixel region “P”. The common electrode 28 is formed on the black matrix 25 and the color filter layers 26 and over an entire surface of the second substrate 22. The arrangement of the liquid crystal molecules is controlled by an electric field between the pixel electrode 18 and the common electrode 28 such that an amount of transmitted light is changed. As a result, the LCD device displays images.
Though not shown in FIG. 1, to prevent the liquid crystal layer 30 being leaked, a seal pattern may be formed along edges of the first and second substrates 12 and 22. First and second alignment layers may be formed between the first substrate 12 and the liquid crystal layer 30 and between the second substrate 22 and the liquid crystal layer 30. Polarizer may be formed on at least an outer surface of the first and second substrates 12 and 22.
Moreover, the LCD device includes a backlight assembly on an outer surface of the first substrate 12 to supply light to the liquid crystal layer 30. When a scanning signal is applied to the gate line 14 to control the TFT “Tr”, a data signal is applied to the pixel electrode 18 through the data line 16 such that the electric field is induced between the pixel and common electrodes 18 and 28. As a result, the LCD device produces images using the light from the backlight assembly.
FIG. 2 is a cross-sectional view showing a conventional array substrate for an LCD device. As shown in FIG. 2, a first metal layer (not shown) is formed on a substrate 59 and is patterned through a first photolithography process to form a gate electrode 60 and a gate line (not shown). The gate electrode 60 is connected to the gate line (not shown). A gate insulating layer 68 is formed on the gate electrode 60 and the gate line (not shown). Next, an intrinsic amorphous silicon layer (not shown) and an impurity-doped amorphous silicon layer (not shown) are sequentially formed on the gate insulating layer 68 and patterned through a second photolithography process to form a semiconductor layer 70 having an active layer 70a and an ohmic contact layer 70b. The semiconductor layer 70 corresponds to the gate electrode 60. A second metal layer (not shown) is formed on the semiconductor layer 70 and patterned through a third photolithography process to form a data line 73, a source electrode 76 and a drain electrode 78. The data line 73 crosses the gate line (not shown) to define a pixel region “P”. The source electrode 76 extends from the data line 73 and is spaced apart from the drain electrode 78. The source and drain electrodes 76 and 78 correspond to the semiconductor layer 70 and cover both end portions of the semiconductor layer 70, respectively. The gate electrode 60, the gate insulating layer 68, the semiconductor layer 70, the source electrode 76 and the drain electrode 78 constitute the TFT “Tr”. A passivation layer 86 is formed on the data line 73, the source electrode 76 and a drain electrode 78. The passivation layer 86 is patterned through a fourth photolithography process to form a drain contact hole 80. The drain contact hole 80 exposes a portion of the drain electrode 78. Next, a transparent conductive material layer (not shown) is formed on the passivation layer 86 and patterned through a fifth photolithography process to form a pixel electrode 88 in each pixel region P. The pixel electrode 88 is connected to the drain electrode 78 through the drain contact hole 80.
Each photolithography process includes a step of forming a material layer, a step of forming a photoresist (PR) layer on the material layer, a step of disposing a mask over the PR layer, a step of exposing the PR layer using the mask, a step of developing the PR layer to form a PR pattern, a step of etching the material layer using the PR pattern as an etching mask to form the gate line, the data line, the gate electrode, a semiconductor layer, and so on. Since each photolithography process requires the mask, the photolithography process may be referred to as a mask process. The mask process requires apparatus for the depositing step, the exposing step, the developing step and the etching step. Accordingly, the mask process causes increase of production costs.
Recently, a method of fabricating an array substrate with less mask processes is introduced. For example, a fabricating process of a contact hole by an in-plane printing method does not require the mask process.
In the in-plane printing method, an organic material, such as photoacrylate, is coated to form an organic material layer. The organic material layer is depressed by a mold having a convex pattern. The organic material depressed by the convex pattern is squeezed such that the contact hole corresponding to the convex pattern is formed. Then, the organic material layer is cured by irradiating UV light. However, there are some problems in the in-plane printing method using photoacrylate.
FIG. 3 is a schematic view showing structure of photoacrylate after photopolymerization. In FIG. 3, a photo-initiator 94 activates polymer 90 such that the polymer 90 and an x-linker 92 are cross-linked to form a branched chain structure. The polymer 90 functions as a main chain 90a, and adjacent main chains 90a are linked by the x-linker 92. The polymer 90 has a pre-determined molecular weight and does not participate in the cross-linking reaction. The polymer 90 does nothing but be cross-linked by the x-linker 92. Accordingly, the photopolymerized polymer has an excellent elasticity due to the above branched chain structure. Unfortunately the excellent elasticity causes a poor hardness. The poor hardness causes damages on a surface of the organic material layer during the in-plane printing method. Moreover, when rubbing process is performed on the passivation layer, which is formed of the organic material by the in-plane printing method and exposed by the pixel electrode, damages are also generated because of the poor hardness. Furthermore, when contacting with rubbing clothes, the organic material layer may be peeled by friction with the rubbing clothes because of the poor hardness.